One field of application for the invention is fixing bones in a desired position, for example to aid in healing of breaks or in the treatment of scoliosis or otherwise to correct abnormal curvatures of the spine. Other bone deficiencies and abnormalities may also benefit from the invention.
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, each having an anterior part, which is the vertebral body, and a posterior part, which is the vertebral arch (or neural arch), the anterior and posterior part enclosing the vertebral foramen. Each vertebral arch is formed by a pair of pedicles and a pair of laminae, and transverse processes and/or a spinous process (or neural spine) project therefrom.
The transverse processes and the spinous process project opposite to the vertebral foramen.
When the vertebrae are articulated with each other, the vertebral bodies form a strong pillar for the support of the head and trunk, and the vertebral foramen constitute a canal for the protection of the spinal cord (or medulla spinalis). In between every pair of vertebrae, there are two apertures, the intervertebral foramina, one on either side, for the transmission of the spinal nerves and vessels.
If the spine of a person has abnormal curvature, the vertebrae are typically inclined relative to one another and relative to said vertebral axis. The lateral edges of the vertebrae on one side are therefore closer together and define a concave outline, while the lateral edges on the other side are farther apart and define a convex outline.
In order to straighten the vertebral column as a remedy for this situation, the lateral edges of the vertebrae on the concave side can be moved away from one another and supported at distances from one another substantially equivalent to the distances between the lateral edges on the convex side. To do so, different kinds of devices may be used.
One such device is the system for fixing a bone to a rod disclosed in international patent application publication WO 2009/047352. It comprises a rod, a blocking body and a conformable elongate member adapted to surround the bone, i.e. to form a loop around it. When using such a system, the elongate member is passed around said bone and through the blocking body and the rod is loaded into the blocking body. The ends of the elongate member are pulled so as to apply tension to the elongate member, and the elongate member and the rod are simultaneously fastened to the blocking body by means of the same fastening system, portion(s) of the elongate member being clamped between the rod and the blocking body. The elongate member and the rod are fastened to the blocking body in one single step.
Another device with the same purpose is described in international patent application publication WO 2011/012690. This device comprises a main body, a flexible elongate member for fixing the main body to an osseous structure, a first fastening member for fastening a portion of the rod to the main body, and a second fastening member for fastening the free end portions of the elongate member to the main body, wherein said second fastening member is distinct from the first fastening member.
In another, similar device known in the art, also comprising a main body, a flexible elongate member for fixing the main body to an osseous structure, a first fastening member for fastening a portion of the rod to the main body, and a second, distinct fastening member for fastening the free end portions of the elongate member to the main body, said second fastening member is laterally offset with respect to the first fastening member, so that a the flexible elongate member is laterally offset with respect to the longitudinal axis of the rod. While such a device facilitates access to the fixation of the elongate member, any force transmitted by the flexible elongate member will induce a significant torsion torque on the rod or the connection of the rod to the fixing device, respectively.